Methods and devices to replace existing sanitary sewer pipes by pipe bursting are well known in the water and sewer industry. These pipe-bursting methods have been in use for at least 15 years. Laterals, those short lengths of pipes that connect a commercial or residential building to the ‘main’ or collector pipe in the adjacent right-of-way, can be replaced using pipe bursting. Replacement of laterals using pipe-bursting is becoming nearly as common as replacement of the main.
There are many reasons the pipe that makes up the lateral might need replacement. Most often the cause is tree roots that grow into the joints between pipe sections. Over time, as these roots grow thick from their successful search for water, the pipes will begin to crack. The gaps will widen and permit an inflow of ground water into the sanitary system. The first clue that these roots are in the sewer lateral may be in the form of a backup. The homeowner or landlord, who is often legally responsible for the condition of the pipe traversing the property, may call a plumber who can remove the roots, by a process often referred to as ‘roto-rooting’.
While the roots may be removed from the interior of the pipe by this process, and blockage may not occur again for months or years, the existence of the damaged pipe creates a bane for the local sewer district. Upon entering a period of rainfall, the water table will rise and ground water, usually referred to as clear water, will enter into the sewer through the leaks caused by the roots. This water, now mixed with the sanitary flow must be treated as sewage. Unfortunately, rainfall may cover the entire sewer district, and the existence of leaking laterals is more common than not in older areas. For this reason, many municipalities experience sanitary flow beyond their capability to process when even moderate rainfall is experienced. For example, Milwaukee, Wis. regularly discharges untreated sewage into Lake Michigan. Further, the District of Columbia has an average of (70) discharges of untreated raw sewage into the Anacostia River each year. The source of these increased flow rates is always at least partially due to leaking pipes allowing ground water to enter the system.
The device and method of the invention are an improvement on a basic concept used in pipe bursting. A cyclic winch applies tension to a wire rope. The rope is further attached to a pipe-bursting device. The device or “mole” may be a simple conical shape, an air-actuated impact tool, a hydraulically actuated device that expands radially upon demand, or when slitting steel, it may be a bladed slitter using can-opener like blades to separate the pipe and allow subsequent expansion. The known process calls for the simultaneous installation of High-Density Polyethylene pipe (hereafter HDPE) or similar product pipe. The product pipe is towed behind the pipe-bursting device.
While cyclic winch devices of this type currently exist, and have existed for more than a decade in the pipe bursting field, such use either completely hydraulically induced grip forces, or in some designs, the grip is entirely self-actuating. In the case of the completely hydraulically induced grip device, the magnitude of the grip on the rope is independent of the pulling force. For this reason, the grip force applied on every cycle must be capable of keeping the rope from slipping should maximum rope tension be needed. This full grip force is applied whether the current job conditions require it or not.
As the grip force applied to the rope perpendicular to the rope's axis is generally 4 to 6 times the tension force, each gripping cycle can cause fatigue damage to the rope's individual wires.
A seemingly improved alternative is to use self-actuating clamp. In this case, friction-inducing teeth, which are actually grooves with sharp edges located on the clamping surface, will start the gripping process. This self-actuating device, which may be either a pair conical collets, wedge shaped, or a four bar linkage, creates a gripping force that is proportional to the axial rope tension. With such a device, the grip force, while still 4 to 6 times the ropes axial tension, need only go high enough to keep the grip from slipping. These devices have two weaknesses. During initiation, if dirt, grease or oil is present on the collet or rope surface, there may not be enough friction to induce or initiate the self-gripping process. Dirt, grease and other slick matter that may cause the rope not to initiate a grip are all substances that exist at the bottom of a sewer pipe. It should be assumed they will coat the wire rope and likely affect the dependability of the grip initiation process.
Further, the sharp edged grooves or teeth that help to initiate the grip when the cable tension is low, will damage the rope when loads are high. When the axial pull force is very high, the similarly high grip force may tend to cut the rope at locations where the collets have sharp features to initiate the process. Thus, what initially seemed like a good technical alternative, has functional weaknesses. The present invention addresses these weaknesses.